Pyrolysis of hydrocarbons

ABSTRACT

A PROCESS FOR THE PRODUCTION OF ACETYLENE AND ETHYLENE BY PYROLYSIS OF LIQUID HYDROCARBONS HEATED IN A FLAME WITH IMPROVED YIELDS BEING OBTAINED THROUGH THE USE OF A KNALLGAS FLAME AND THE INTRODUCTION OF THE HYDROCARBONS IN A FINELY DIVIDED OR DROPLET FORM.

United States Patent ,704,332 PYROLYSIS 0F HYDROCARBONS Olle B. Lindsn'om, Lorensviksvagen 14, Taby, Sweden No Drawing. Filed Nov. 6, 1970, Ser. No. 87,614 Claims priority, application Sweden, Nov. 7, 1969, 15,354/69 Int. Cl. C07c 3/00 US. Cl. 260-679 R 3 Claims ABSTRACT OF THE DISCLOSURE A process for the production of acetylene and ethylene by pyrolysis of liquid hydrocarbons heated in a flame with improved yields being obtained through the use of a knallgas flame and the introduction of the hydrocarbons in a finely divided or droplet form.

This invention relates to a process for the production of acetylene and ethylene by pyrolysis of liquid hydrocarbons heated in a flame.

Pyrolytic decomposition of hydrocarbons has been effected industrially by various processes and for various purposes. For example, pyrolysis is a frequently utilized method for the preparation of acetylene and ethylene. In the preparation of acetylene, it is necessary rapidly to bring the starting material, which may be constituted by various hydrocarbons ranging from methane to heavy fuel oils, to temperatures above 1000 C. Unsaturated hydrocarbons, such as acetylene, are generated quickly at these high temperatures by reaction of free radicals that form in the course of the pyrolysis. It is likewise characteristic of these methods that there is an eflort to cool the reaction mixture very rapidly to temperatures below 400 C. to avoid decomposition, especially of the acetylene.

The rapid heating of the starting material to temperatures above 1000 C. may be achieved with various known combustion processes and with various electrical processes. As is known, extremely high temperatures can be attained in a plasma and in electric arcs. The electrical methods generally afford comparatively high yields of acetylene, since the acetylene equilibrium is favored by the very high temperatures that can be attained. The electrical methods require, however, a great energy consumptionoften about 10 kw. h./kg. acetylene. The are and plasma methods have many variants, and are generally characterized by various processes and devices used for stabilizing the reaction zone.

Direct electric resistance affords a stable and well defined reaction zone. However, carbon deposited on the heating surfaces is detrimental to heat transfer.

One of the prior art regenerative processes, which is known as the Wulff process, is characterized in that the gas is decomposed by contact with elements that are heated to 1100-1300 C. in a preliminary heating step. The heating can occur by the combustion of a suitable gaseous fuel which may have a composition other than that of the gas that is to be decomposed. The reaction gas is cooled rapidly after contact with the hot elements. One problem with this method is to effect a satisfactory rapid cooling to below 400 C.

A further way to attain the desired high temperatures is by the combustion of the starting material, whereby the heat is delivered simultaneously with the decomposition. The combustion can be maintained with oxygen. Many methods have been described in the literature and applied commercially that are based on partial combustion of hydrocarbons. The temperature in these flames does not approach the high values of the plasma method, for

which reason the acetylene yield is lower. Moreover, quite large quantities of carbon monoxide and hydrogen are formed as by-products.

Combustion and pyrolysis can occur in two consecutive stages, with a first combustion gas stage that gives heat to a second decomposition stage. With this method, a cheap hydrocarbon can be used as fuel, while a more reactive hydrocarbon can be utilized for the decompositiOn. Pyrolysis in a diffusion flame can be said to be a combination of the two above-described methods, part combustion alone, and part combustion with subsequent pyrolysis. In this case, it is also possible to use a cheap fuel for heat generation for the endothermal cracking of the hydrocarbon, whereby at the same time a part of the hydrocarbons is also burned. The gases can be fed into the flame through separate concentric rings. A combustion gas zone is formed between the oxygen and the fuel, and the heat is transferred first to the hydrocarbon that issues from the outermost ring.

The desired fast cooling after pyrolysis is effected in general in such a way that the reaction gas is brought into contact with a colder liquid phase in that the flame is burned in or near it, and in that the reaction gases are brought into direct contact with a spray or drops of a liquid phase. The liquid phase can be water, a molten salt or frequently a hydrocarbon that is used as starting material for the decomposition and/or the combustion.

A very suitable and near to hand combination of heating and cooling processes is the so-called submerged combustion, i.e., combustion in the liquid phase. These processes are characterized in that the combustion and pyrolysis takes place in a flame that burns underneath a homogeneous liquid phase. A variant of these processes is partial combustion of oil with air in a flame that burns below the surface of the liquid. Carbon monoxide can also be supplied as a combustible gas in this case, to avoid the formation of water in the reaction zone. In some of these methods, the flame is ignited with hydrogen. When the combustion has become well established, the delivery of hydrogen is cut off, whereafter oxygen alone is supplied.

A method that for the present is the object of great interest works with two different streams of hydrocarbons. The combustion nozzle comprises three concentric rings. Through the middle one, oxygen is delivered, or oxygen containing gas, for the combustion. The soot that forms in the combustion accumulates in the hydrocarbon phase that the flame burns in. The soot-charged hydrocarbon is pumped from the vessel and divided into two streams, one of which is filtered and fed into the innermost of the said three rings, while the other is cooled and fed to the outermost ring. In this way, the sooty parts of the hydrocarbon phase are utilized as fuel and also for some production of acetylene. The main part of the gaseous product is formed by pyrolysis of the filtered stream, however.

With the introduction of liquid hydrocarbon into these flames there is, of course, first formation of drops, and then a rapid evaporation of the liquid hydrocarbon. It has also been proposed that the liquid hydrocarbon be introduced by means of sprayers to attain a fine division of the liquid phase in the reaction zone, thereby to have more rapid heating.

It has also been proposed that the last-described process be modified by feeding gaseous fuel, especially the residual gas that remains after separation of the desired reaction products, e.g., acetylene. To this residual gas, which thus is utilized as fuel, there can be added other gaseous fuels such as refinery gas, coke oven gas, water gas or gas that is produced by carburization of carbon according to various methods. A great part, often half, of the fuel gases in this case is composed of carbon compounds.

It is an object of the present invention to provide an improvement to the last-mentioned process scheme. In accordance with the invention, it has been found that very good yields, especially of acetylene and ethylene, are obtained if a liquid hydrocarbon is introduced in finely divided form into a knallgas flame, fed with a fuel gas that contains more than 80%, advantageously more than 95%, oxygen, whereafter the reaction gas is cooled in direct contact with a liquid phase. An important novel feature in this process resides in the fact that drops of the liquid hydrocarbon are heated very rapidly in a medium that partly has a very high temperature and good heat conductivity, and partly has a substantially lower concentration of carbon containing compounds than in the former methods because of the composition of the fuel gas. How these factors cooperate for the surprisingly good reaction yield when pyrolysis is effected according to the invention are not clearly understood.

The new process of the present invention may be carried out by a combination of known processes. For example, it is possible to effect the pyrolysis entirely by means of the devices that are described in US. Pat. No. 2,985,695, where a gas is used as fuel, the said gas containing more than 80% by volume hydrogen, advantageously 95% by volume. The oxygen-containing gas is to contain more than 80% by volume, advantageously more than 95 by volume oxygen. Only under these conditions that were not previously described is there the characteristic reaction course of the present invention, with unexpectedly high yields of acetylene and ethylene.

With reference to the economy of the process of the present invention, it is advantageous to prepare the hydrogen for combustion from the residual gas of the process by known separation techniques. The carbon mon oxide content of the residual gas may be converted to hydrogen with subsequent separation of the hydrogen that is formed.

Hydrogen may also be provided in a known manner from the slurry of carbon and coke products in the hydrocarbon phase that is continuously separated from the hydrocarbon content of the reaction vessel, whereby this hydrogen is also fed to the fuel gas of the process.

However, it is not necessary, as in the above-cited patent, to effect the desired fast cooling of the reaction gas since the knallgas flame is disposed in a liquid gas, advantageously a liquid hydrocarbon. Good yields are also attained if the flame is directed in a known way toward a liquid surface or if the flame burns in a reaction chamber in which a spray, liquid film, or liquid drops are introduced for direct and rapid cooling of the reaction gases.

The knallgas burner, one or more of which may be disposed in the reaction chamber, may be provided with known devices for producing a fine division of the liquid hydrocarbon. The hydrocarbon can also be finely divided and fed to the reaction zone by separate devices, e.g., the kind used for injection of liquid hydrocarbon into combustion chambers, etc. From what has been said, it is evident that there is no difliculty for the person having ordinary skill in this art to devise a suitable process system for effecting the pyrolysis process of the invention.

As has been mentioned above, the hydrogen gas that is present as a by-product in the process may advantageously be restored and utilized in the process. The carbon monoxide that is formed as a by-product may also be converted advantageously to hydrogen and carbon dioxide, whereby this hydrogen is also fed to the knallgas flame. The proportion between the fuel gas that is delivered, suitably prepared in this way, and the oxygencontaining gas, is a function of the stoichiometry of the knallgas reaction. For each delivered gram molecule hydrogen there is thus delivered approximately one-half gram molecule oxygen. The proportion of hydrocarbon and fuel gas varies, depending upon the composition of the hydrocarbon, the configuration of the reaction chamber and the burner, the method of cooling, etc. If the cooling is effected with use of a liquid hydrocarbon, which often is advantageous, this can also take part in the pyrolysis to a certain extent, which, of course, affects the combustion gas requirements. In practice, it is relatively easy to balance the various process flows to attain anoptimal result with respect to composition of the reaction gases, the yield of desired compounds, and the economy.

The following figures for the process variables are therefore to be taken as typical values of one specific example.

The dry producer gas often contains between 5 to 10% by volume acetylene and between 5 to 10% by volume ethylene. The above components in the producer gas comprise for the most part approximately equal propor tions of carbon monoxide and hydrogen, with traces of carbon dioxide, hydrocarbons other than acetylene and ethylene, unburned oxygen, etc. The conversion of the carbon monoxide to hydrogen thus affords an excess of hydrogen which corresponds by volume to the reacted carbon monoxide. From these summarizing figures, it is evident that the quantity of hydrogen that is to be delivered to the flame for preliminary combustion will, in general, amount to a volume that is not much below the volume of the dry producer gas. The delivered oxygen gas volume will thus comprise about half the hydrogen gas volume. Consumption of oxygen is less, calculated on the produced amount of acetylene and ethylene, compared with partial combustion. Since hydrogen is generated in the pyrolysis, there is a reaction medium in the flame that is characterized by an excess of hydrogen, contributing favorably to the desired result, when pyrolysis is effected according to the present invention.

In terms of weight, the proportions naturally are dif' ferent. Approximately 70 to 85%, generally about 80%, of the crude hydrocarbons react to form acetylene and ethylene, while the rest goes to generation of hydrogen that is utilized as fuel in the process.

The relationships in the reaction zones formed in the diffusion flames of the invention evidently differ from thoserof flames with partial combustion of the hydrocarbon according to earlier techniques. As reaction conditions most closely correspond to those present in plasma and are method, the method of the invention might be described as a chemical plasma method. It has also proved to be advantageous, for purposes of stabilizing the flame, to maintain an electric are or to introduce a plasma in conjunction with the burner. The amounts of energy that are required are supplied electrically. However, substantially less is required than the combustion heat generated by the flame.

The invention will be further illustrated in the following, the result of a half-scale experiment.

The pyrolysis took place in a knallgas flame that was supplied with l mole/sec. O and 2 moles/sec. H The flame burned in a heat-insulated 0.3 m. reactor. Cooling of the pyrolysis gas occurred by means of a sheet of flowing water at about C. By means of a sprayer, 0.25 mole/sec. heptane was introduced into the flame. The dry pyrolysis gas was analyzed by gas chromatography and was found to contain 0.33 mole/sec. C H 0.03 mole/sec. C H 0.13 mole/sec. CH 0.87 mole/sec. CO and 1.3 mole/ sec. H as well as traces of CO and O Conversion of CO yielded a surplus for hydrogen gas production, so that the hydrogen requirement of the process was completely covered.

The present embodiments of this invention are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

I claim:

1. In a process for the production of acetylene and ethylene by pyrolyzing liquid hydrocarbons by heating of the hydrocarbon in a flame and cooling a reaction gas by bringing the reaction gas into direct contact with a liquid phase, the improvement wherein in the step of pyrolyzing the liquid hydrocarbon, the hydrocarbon is fed to a knallgas flame, a fuel gas for said knallgas flame comprising more than 80 percent by volume hydrogen and an oxygen-containing gas comprising more than 80 percent by volume oxygen, and the hydrocarbon being fed to the flame in tne form of drops.

2. The process as defined in claim 1 characterized in that the fuel gas for the knallgas flame contains more than 95% by volume hydrogen and the oxygen-containing gas contains more than 95 by volume oxygen.

3. The process as defined in claim 1 characterized in that the fuel gas for the knallgas flame is recovered from the gas that remains after recovery of acetylene and ethylene from the reaction gas.

References Cited UNITED STATES PATENTS 2,785,054 3/ 1957 Bethea et al 23209.4 3,595,618 7/1971 Kiyonaga et a1 23209.4 2,453,440 11/1948 Kaufmann et a1. 23209.4 2,985,695 5/1961 Platz et al. 260679 1,995,136 3/1935 Winkler et a1. 260170 1,864,196 6/1932 Herrmann et al. 260679 2,343,866 3/1944 Hincke 260679 1,896,552 2/ 1933 Millar 260679 DEL-BERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner US. Cl. X:R. 260683 -R 

